Remove trailing commas for IE 9 compatibility

Summary: IE 9 doesn't like trailing commas. When we introduced eslint, we added
a bunch of trailing commas, which makes IE 9 sad.

Test Plan:
 - `make lint`
 - Visit http://localhost:7936/ using IE 9 on browserstack.
 - See that the math loads, and there are no errors in the F12 developer tools.

@kevinb

server.js

@@ -64,7 +64,7 @@ app.get("/katex.css", function(req, res, next) {
 
         less.render(data, {
             paths: [path.join(__dirname, "static")],
-            filename: "katex.less",
+            filename: "katex.less"
         }, function(err, output) {
             if (err) {
                 console.error(String(err));